A passive communication system refers to a system including a device being wirelessly powered up and being passively operable, without its own power, for data transmission and reception (hereinafter referred to as a “passive device”) and a device which supplies power to the passive device wirelessly for data transmission and reception (hereinafter referred to as an “active device”). A passive RFID (Radio Frequency IDentification) communication system, being representative of a passive communication system, is a system in which a passive device (passive RFID) is attached to a particular physical object and stores data related to the object, such as an identification code and so on, and an active device (a RFID reader) supplies power to the passive device wirelessly and collects information on the object.
In recent years, a passive communication system has been proposed to transmit high-capacity multimedia data as well as low-capacity data such as an identification code at a high speed (see, e.g., J. McDonnell et al, “Memory Spot: Labeling technique”, IEEE Pervasive Computing, vol. 9, no. 2, pp. 11-17, April-June 2010.).
However, such a passive communication is disadvantageous in transmitting high-capacity data at a high speed since a passive device has to be operated with a small quantity of power and is unstable in its power supply state due to dependency on power supplied wirelessly, a frequency of a message transmitted by the passive device is unstable due to the limitation in its power supply, and the strength of a signal is so weak to produce a frequent error in message reception.
Thus, in order to avoid such a frequent error in message reception, the passive device limits the length of a message to be transmitted at a time. This is because the passive device has to transmit the message again in the event of an error in message reception and the increase in length of the message results in increase in probability of error in message reception. However, a long message has to be transmitted at a time for the purpose of effective transmission of high-capacity data at a high speed.
FIG. 1 shows a structure of a message in a typical passive communication system in which, for a data transmission request 10 from an active device, a passive device transmits a synchronization signal 11, actual data 12 and a checksum 13 used to detect a data transmission error to the active device.
For transmission of one message 14, the passive device consumes time required for actual data transmission, time required for reception of the data transmission request from the active device, and time required for transmission of message elements (a synchronization signal for message synchronization between the active device and the passive device, and a checksum used to determine whether or not data are correctly received) additionally required for actual data transmission.
However, such a passive communication system has a very unstable link frequency and is forced to provide a short message to avoid a high probability of error with the elongation of message. However, this system has a problem of low data transmission efficiency since time required in addition to the time required for the actual data transmission is relatively increased with the short message.
As an attempt to avoid this problem, some active communication system (e.g., R. K. Ganti et al, “Data Streaming in Wireless Sensor Network”, Proc. International Conference on Embedded Networked Sensor Systems, 2006, pp. 209-222) use a division checksum method in which one long message is divided into a plurality of message fragments 23 and 24, as shown in FIG. 2, and a number 21 and a checksum 22 for identifying an order of message fragments are inserted for each message fragment. This division checksum method provides high reception efficiency since only message fragments with an error are re-transmitted and received without re-transmission of the whole message even if a reception error occurs in some message fragments.
However, application of such a division checksum method to a passive communication system provides a less effect than an active communication system. Since the passive communication system has very unstable frequency of a message to be transmitted, as opposed to the active communication system, if an active device receives a message having an unstable frequency transmitted by a passive device, synchronization with the message is likely to be lost and messages cannot be correctly received after the point of time when the synchronization is lost.
In this case, even when the message is divided into a plurality of message fragments, message fragments received after the point of time when the synchronization is lost cannot be correctly received at all, which has no contribution to improvement of transmission efficiency.
As an attempt to avoid the above problem of unstable frequency in the passive communication system, US Patent Publication 2007/0035383 A1 (titled “Radio Frequency Identification Interrogation Systems and methods of Operating the Same”) discloses a method in which 3 synchronization signals are respectively inserted in the beginning, middle and end of a message in passive RFID, as shown in FIG. 3. This method uses several synchronization signals 31, 32 and 33 to alleviate losing of synchronization in message reception.
However, using the fixed number of synchronization signals cannot respond suitably to a variety of communication states. For example, if a passive device transmits a sufficiently strong signal with a correct frequency, transmission of several synchronization signals consumes wasteful time. If a passive device transmits a weak signal with a very unstable frequency, three synchronization signals are insufficient to alleviate losing of synchronization.
In addition, even with alleviation of losing of synchronization, this method has another problem of re-transmission and reception of the whole message in the event of an error in message reception.